[scudo] Only prepare PageMap entry for partial region

[scudo] Only prepare PageMap entry for partial region

This reduces the size of PageMap and we are more likely to use the
static local buffer. Note that now this is only supported for single
region case, i.e. on SizeClassAllocator64. For SizeClassAllocator32,
it needs a different way to save the PageMap.

Differential Revision: https://reviews.llvm.org/D142659